1. Field of the Invention
This invention relates to a solid state radiographic imaging device and more particularly to a device having a plurality of sensors with extended active image capturing surface area.
2. Description of Related Art
Advances in solid state electronic component technology have led to the development of image capture panels comprising two dimensional arrays of individual sensors. Such panels have found applications in radiography wherein the panel is exposed to modulated X-ray radiation carrying imaging information. The image information is stored in the individual sensors typically in the form of charges trapped in a capacitor. These charges which represent the image are read out of the array capacitors and following electronic processing, the charges are stored for display as a visible image. When the radiation is X-ray radiation which impinges on the panel after passing through a patient, the end result is a medical radiogram.
U.S. Pat. No. 5,319,206 issued to Lee et al on Jun. 7, 1994 describes a system which employs such a panel to capture a radiogram.
The panels, as stated above, comprise a two dimensional array of sensors with associated switching and addressing circuitry built on an insulating substrate, usually a glass plate. Such sensors typically include a pair of generally coplanar conductive microplates separated by a dielectric layer. Extending over all the sensors above the microplates is a photoconductive layer which is sensitive to X-ray radiation. A top electrode is placed over the photoconductive layer.
The two microplates in each sensor serve to collect and store charges representing the radiation exposure of the sensor. Radiation exposure is the product of the radiation intensity and the time duration of radiation impingement on the sensor.
In operation, a charging voltage is applied to the bottom microplates of all sensors and the top electrode. This creates an electric field in the photoconductive layer. Upon exposure to radiation, electron/hole pairs are generated in the photoconductive layer by the absorbed radiation exposure energy. Under the influence of the applied electric field the electrons and holes produced separate and migrate along the field lines toward the top electrode and toward the microplates. In detector structures where a positive charging voltage is applied to the top electrode, electrons move along the field lines toward the top electrode, and holes migrate toward the top microplates. The hole migration results in a charge accumulation during exposure in the charge storage capacitors formed by the two microplates and the dielectric separating them. Subsequent removal of the charging voltage and the exposing radiation leaves the accumulated charges trapped in the capacitors.
As can be seen by this brief description of the panel operation, the charges are captured in the sensor areas covered by the microplates. These areas are typically confined by orthogonal intersecting columns and rows of interstitial spaces in which run conductive strips which are used to address the individual sensors, to recover the stored charges during readout of the image, and to apply the charging voltage to the bottom microplates.
In order to individually address each sensor, a solid state switch is built in each sensor. One side of the switch is connected to the top microplate. A typical addressing and switching arrangement uses a TFT transistor switch constructed in a cutout portion of the microplates. The technology to produce TFT transistor switches is disclosed in U.S. Pat. Nos. 5,003,356 and 5,032,883.
The area occupied by the TFT transistor, the charging lines, the data strips and the addressing strips are non imaging areas. It is therefore important to minimize them. However very little can be done to minimize the area occupied by the switching element, and there are practical limits as to how close to the data and address strips can the microplates be built without risking shortages.
The prior art has attempted to extend the image capture area by a sensor structure as shown in FIGS. 1 and 2 which are a schematic cross-sectional elevation and a top view of a typical prior art sensor.
As shown in FIG. 2, the sensor includes a substrate 9, which may be a glass plate, a bottom microplate 12 and a top microplate 14. Microplates 12 and 14 are separated by a dielectric layer 13. Each sensor includes a TFT transistor switch 15. The TFT switch has a gate electrode 16 built on the glass substrate 9. The TFT gate electrode is covered by an insulating layer which can be the same as dielectric layer 13. In addition to the gate electrode, it includes a drain electrode 18, a source electrode 20 and a semiconductor material 19. The drain electrode 18, is the electrode which receives the data signal from the capacitor, and is directly connected to a top microplate 14.
As shown in FIG. 1, the source electrode 20 is connected to a data conductive strip 22 extending along one side of the sensor, while the gate electrode 16 is connected to an addressing conductive strip 24 running along a second side of the sensor. A photoconductive layer 30 and a top electrode 32 are coated over the microplates, the TFT switch, and the addressing and switching strips as shown in FIG. 2. Additional insulation layers not illustrated are often used between the photoconductor and any conductive electrodes in the panel to prevent direct contact between the conductors and the photoconductor layer.
The prior art extends the active image capture area of the first top microplate 14, by using a second top microplate 26 to create a composite top microplate structure. The first top microplate 14 is totally covered by the second top microplate 26. The surface area of the second top microplate 26 is larger than the first and the second top microplate rises above and extends like a mushroom or tent over the TFT switch. As shown in FIG. 2 the second top microplate also extends to cover a portion of the spaces between sensors.
This solution, albeit effective in increasing the active area of the sensor, has the unwanted side effect of shielding the top microplate 14. Because in erasing the sensor following image readout, the array is illuminated with uniform visible radiation which renders the photoconductive layer conductive and results in the redistribution of the charges, it is important that as much light as possible reaches the photoconductive layer in as uniform a manner as possible. Passing through two layers of deposited metal, no matter however transparent, results in some illumination loss. It is therefore desirable to provide a top microplate structure which, while maintaining the efficiency of the dual microplate extended active surface area for charge capture, does this without loss in visible light transmission during the erase cycle.